In view of the increasing data rate per time in the conventional radio communication system, a multi-level modulation technique for propagating information of multiple bits per symbol has been developed. In such a multi-level modulation technique, the maximum throughput increases, when the quality of the propagation paths is further improved, as the number of bits per symbol increases. On the other hand, the throughput largely decreases, when the quality of the propagation paths is lowered, as the number of bits per symbol increases. Therefore, an adaptive modulation technique has been proposed to switch the modulation levels according to the quality of the propagation paths so as to realize stable communication. This technique has been described in the paper titled “Transmission Characteristic of Modulation Level Variable Adaptive Modulation Type” (B-II Vol. J78-B-II No. 6 pp. 435-444, June 1995 of The Transactions of the Institute of Electronics, Information and Communication Engineers) (Non-patent document 1).
Moreover, with the frequency band of radio communication expanding, the OFDM (Orthogonal Frequency Division Multiplexing) system for transmitting information by dividing a transmission into many orthogonal subcarriers has been used, and the OFDM adaptive modulation type system to switch the modulation type for each subcarrier has also been proposed to cover a difference in the quality of the propagation paths within the OFDM bandwidth. This technique has been described in the paper titled “OFDM Adaptive Modulation type utilizing Multilevel Transmission Power Control for High-Speed Data Communication” (B-II Vol. J84-B-II No. 7 pp. 1141-1150, July 2001, of The Transactions of the Institute of Electronics, Information and Communication Engineers) (Non-patent document 2).
Patent Document 1] “Transmission Characteristic of Modulation Level Variable Adaptive Modulation type” (B-II Vol. J78-II No. 6 pp. 435-444, June, 1995, of The Transactions of the Institute of Electronics, Information and Communication Engineers) by Otsuki, et al.
[Patent Document 2] “OFDM Adaptive Modulation type utilizing Multilevel Transmission Power Control for High-Speed Data Communication” (B-II Vol. J84-B-II No. 7 pp. 1141-1150, July, 2001, of The Transactions of the Institute of Electronics, Information and Communication Engineers) by Yoshiki, et al.
Both a transmitter station and a receiver station need to have common information on a modulation type in order for the receiver station to perform correct demodulation during switching of the modulation type according to a variation in the propagation path by the conventional adaptive modulation technique.
If the transmitter station and the receiver station have a different modulation type, the data location is deviated and continuous errors occur. Therefore, in the scheme, for example, where the modulation type is communicated from a transmitter station to a receiver station, the signal carrying this information must always be sent with a higher accuracy. Moreover, even in the case where the receiver station assumes, for example, the modulation type from the received signal, a training signal for highly accurate estimation is necessary.
Therefore, when it is attempted to control the modulation type for each subcarrier in the OFDM system, a problem arises, in that the throughput of the data signal itself is suppressed because the signals for controlling the modulation type, such as the modulation type, indicating signal and the training signal, increase. On the contrary, when the number of signals for controlling the modulation type, such as the modulation type indicating signal and the training signal, is reduced so as to not suppress the throughput of the data signal, a problem also arises in that the degree of freedom in the modulation type control is reduced, fine control becomes impossible, and communication by fully utilizing the propagation path cannot be realized.